1. Field of the Invention
The present invention pertains to the correction of the monitoring signals given by a sonar antenna made up of hydrophones mounted on a wall such as, for example, a hull. In the following description, the term "hull" is used by way of an example, it being understood that whatever is stated about it can be transposed to any wall subjected to vibrations in water.
Hydrophones are acoustic sensors designed to work in water and each hydrophone of the antenna gives a signal that is a function of the incident waves (the term "wave" is used in this description to mean "acoustic waves") coming from the environment external to the hull.
It may be recalled that a known way of finding the direction of an incident wave is to form channels, with each channel corresponding to a determined direction.
2. Description of the Prior Art
With hydrophones evenly spaced out at a distance d and arranged in a line that makes an angle a with a given direction, a channel in a given direction is obtained by phase-shifting the hydrophone signals by a value w.d.sin a/c where w is the pulsation of the incident wave and c is the speed of sound in water. For the channel to be properly formed, the incident wave should be planar at the frequency w/2.pi..
The antenna has to be mounted on a support that is generally the hull of a naval vessel (a surface or submarine vessel). Now, the hull is elastic and therefore does not form a rigid baffle: it vibrates under the effect of the incident wave, and the vibrations of the hull disturb the planeity of the wave. The result thereof is that the phase relationships among the signals received by the hydrophones are disturbed and that the formation of the channels is no longer properly ensured.
For, as is well known, each hydrophone receives the incident wave and the wave reflected on the hull. The reflected wave may be said to consist of a wave that the hull would have given if it had been perfectly still (this is the rigid baffle effect) and a wave due to the vibration induced by the incident wave.
The rigid baffle effect does not hamper the formation of the channels, for the amplitude and the phase of the corresponding waves depend little on the location of the hydrophone on the hull, irrespectively of the frequency and the incidence of the exciting wave. By contrast, the vibration has a disturbing effect.
A known way of correcting for this disturbing effect is to place an acoustic barrier between the hull and the hydrophones of the antenna. This barrier remains fixed under the effect of the incident wave reflected on it.
However, this barrier is ineffective at frequencies of below 1 kHz since it has to withstand the hydrostatic pressure and should therefore be sufficiently rigid and since its dimensions remain smaller than the wavelength of the wave that is picked up.